The modern total hip replacement was developed in the early 1960's by Sir John Charnley, an orthopedic surgeon working at a small country hospital in England. His work significantly contributed to one of the great triumphs of twentieth century surgery. Total hip replacement was first performed in the United States in the late 1960's. Hundreds of thousands of replacements per year have been performed in the United States since the late 1960's. The operation has become fairly routine and is successful in more than ninety five percent (95%) of hip replacement operations.
Total hip replacement involves the implantation of an artificial implant to replace a diseased ball and socket of a patient's hip joint. This is accomplished by replacing the ball (femoral head) at the top of the thigh bone (femur) and the socket (acetabulum) in the pelvis. The implant components are thereby termed the femoral component and the acetabular component. The femoral component is comprised of a head, or ball, and a stem. The acetabular component is comprised of a metallic hemispherical cup with a low-friction liner (e.g., plastic or ceramic or polished metal) or of a hemispherical cup of a low-friction material.
Ball-in-socket motion, simulating the motion of the natural hip joint, occurs between the highly polished head (ball) of the femoral component (usually a chrome metallic alloy (or ceramic material) and the highly polished liner of the acetabular component (usually high strength polyethylene plastic or ceramic). Methods of fixation of the components to bone currently fall into two categories. One utilizes a cement agent (polymethylmethacrylate) pressurized into the bone surrounding the implant. The other utilizes a surface treatment fostering bone ingrowth into the implant.
The major long-term problems with cemented hip replacements include (1) loosening of the bond between the implant and the bone and (2) wear at the interface of motion between the implant parts. If either of the implant components becomes loosened, a second surgery is likely necessary for reimplantation of well fixed components. Currently, the rate of loosening is approximately 0.5% per year (5 of 100 patients after 10 years). Much research has been done, and continues, regarding fixation (and loosening) concerns, both with cemented and bone ingrowth methods. Polyethylene wear has caused great concern regarding the contribution of its wear debris particles to local bone destruction and component loosening. As such, minimizing and measuring polyethylene wear have become major issues in the field of hip replacement.
There are several early term problems that a patient may encounter after total hip replacement surgery. Most are effects associated with any major surgery, such as blood loss, wound infection, blood clots, pneumonia, and heart attack. However, the most common complication inherent to total hip replacement alone is dislocation of the hip. At any time after the surgery, the ball can dislocate from, that is slip out of, its socket defined by the low-friction liner.
Attempts at direct coupling of the ball and the socket have only led to excessively high loosening rates. As such, the ball must be balanced in the socket and maintained there, initially by intrinsic muscular contraction and subsequently by scar tissue. As such, dislocation is more likely early after surgery than later. Currently, the rate of dislocation is approximately 5%. Dislocation usually occurs during the first year. Dislocation occurs in the first year at a rate ten times the rate of component loosening--or at a rate in the first year of approximately 5% of the hip replacement operations.
The ease at which dislocation of the hip occurs depends on multiple factors including technical factors (component position, muscular tension) and rehabilitation factors (position of the leg, muscular strength, patient activities). Despite careful instruction in proper position, exercise, and activities, patients experience dislocation both in hospital and at home. Dislocation causes severe pain, inability to move the hip, inability to walk and deformity of the leg and foot.
If dislocation occurs at home, dislocation requires a trip back to a hospital, often by ambulance. Dislocation must be confirmed by x-ray, as must subsequent relocation. Any necessary treatment to relocate the hip must be done, potentially including a general anesthesia, another surgery, further hospitalization, further physical therapy, and/or an extensive brace. All of these treatments involve significant disadvantages in patient discomfort, added exposure to risk, lost rehabilitation time, and expense.
The value of a detection device for acetabular component liner (or lining) material wear would also serve great value in improving longetivity, and therefore quality, of hip replacement components. The ideal detector would allow quantifiable measurement of material wear at the liner (or lining)of the acetabular component over intervals of time, in turn allowing identification of those hip replacements at risk for the untoward residual of wear debris, those being bone destruction (osteolysis) and component failure due to loosening.
The value of a detection device for hip replacement dislocation would serve great value in improving the quality and decreasing the cost of patient care. The ideal detector could emit an alarm message when dislocation is impending or serve to confirm absence, or presence, of dislocation. The cost of such a detector would be far outweighed by its value in avoiding hip dislocation and in avoiding the patient discomfort, patient risk, and significantly greater expense associated with dislocation. Until now, no satisfactory device for detecting impending hip dislocations exists.